Sounds that a person hears include pleasant sounds and unpleasant sounds. Typical examples of the unpleasant sound are factory noises and transportation noises made by cars, airplanes, and the like. On the other hand, examples of the pleasant sound include music. Here, music may be pleasant for a person who is willing to enjoy this music. However, this music is not always pleasant for any other person present nearby. For example, suppose that audio listening and TV watching are enjoyed in a living room of a house. In this case, these sounds are pleasant for those who listen to the sounds near the audio system and the TV. However, for those who enjoy a conversation in the same living room, these sounds may make the conversation hard to hear and thus disturb the conversation. Although those who enjoy the audio listening and TV watching would like to enjoy the sounds at high-volume levels, the volume levels just have to be turned down because the sounds may interrupt the conversation. This leaves much to be desired by those who enjoy these sounds. Here, suppose that elderly man and woman are watching the TV. In general, the elderly have reduced hearing ability and, for this reason, the elderly tend to turn up the volume level considerably high. As a result, the sound from the TV becomes noise that increasingly interrupts the conversation, and this may possibly lead to a family problem.
In order to solve such a problem, the sound may be reproduced (outputted) only in an area for listening to, for example, TV and thus may not be reproduced in other areas. One of the most typical conventional techniques to solve the problem is to use a directional speaker. Classical examples include a geometrically-shaped speaker, such as a horn speaker. With the geometric form, it is relatively easy to obtain directivity at high frequencies. At low frequencies, however, the diameter and depth needs to be long in order to obtain sharp directivity, thereby increasing the size of the speaker. With this being the situation, in recent years, techniques of a parametric speaker (an ultrasonic speaker) and an array speaker may be used. The parametric speaker demodulates an original audio signal in air from ultrasound modulated by an audio signal, using nonlinearity of air with respect to ultrasound. The array speaker obtains directivity by the synthesis of sounds radiated from a plurality of speakers arranged linearly.
However, any of the above directivity control techniques performs control to allow a reproduced sound to propagate in a certain direction (the front direction of the speaker, for example). This means that when a person is present in this direction regardless of whether the person is ahead (in front) of the speaker or behind the speaker, the sound is transferred and thus heard by this person. To be more specific, by strongly controlling the directivity into the front direction of the speaker, it is possible to make it difficult to hear the sound in the right and left directions. However, it is impossible to perform control to allow the sound to be heard only by a person present in front of the speaker in the front direction and not to be heard by a person present behind the speaker in the front direction. In other words, the reaching distance of the sound cannot be controlled. In order to solve this, in a gallery or a museum for example, a directional speaker is fixed to the ceiling to limit the reproduction area to the front of a subject of appreciation, like spotlighting. However, in the case of the environment to watch TV, since the TV screen is located in front of a viewer, the sound needs to be reproduced from the front of the viewer. Otherwise, the image on the screen and the reproduced sound image do not agree with each other, thereby causing extreme discomfort. This can also be said to the case of audio listening. More specifically, it is the most common and pleasant to reproduce the sound image in front of a listener. This is because, since the original sound field of music recorded into a sound source such as a compact disc (CD) is a concert hall or a studio, the original sound field and sound image need to be reproduced to recreate the presence as if the orchestra were actually present at the location.
On account of this, it is desired for the speaker to be placed in front of the listener. The problem described above cannot be appropriately solved by the conventional directivity control technique.
Moreover, in the case of the directivity control technique, a geometrically-shaped speaker such as a horn speaker, a plurality of speakers, or an ultrasound device is used. Thus, a problem arises, for example, that the size of the speaker increases in order to control the sound at low frequencies or that the low-frequency control is difficult. For this reason, the directivity control technique is usually employed for the high-frequency control.
Furthermore, research and development have been conducted on a technique that recreates any sound field by sound field control fully employing signal processing. One of the examples is the boundary sound field control technique employing the Kirchhoff-Helmholtz integral equation. This method controls a sound pressure and sound pressure gradient (sound particle velocity) on the boundary surface in a certain enclosed space to faithfully recreate the original sound field in a different enclosed space having the same form. This method has an advantage in the low-frequency control. However, it is difficult for the method to perform the high-frequency control. Moreover, a problem arises that, for example, since the system scale increases in order to perform the high-frequency control, it is difficult to implement broadband control.
The problems and measures of the directivity control technique and the boundary sound field control technique are disclosed in Patent Literature 1, Patent Literature 2, and Patent Literature 3. Moreover, Patent Literatures 4 and 5 and Non Patent Literatures 1 to 5 also disclose the measures and the like for sound field control.